The present invention relates to a syringe, and in particular, to a mechanism for locking a needle inside a hollow shield to prevent repeated use.
Contaminated needles have become increasingly dangerous to people in the medical profession due to the increase in people infected with the AIDS virus and other infectious diseases. Therefore, various syringes have been developed to cover the needles after use. Often, the needle retracts to reduce the risk of pricks by an exposed contaminated needle.
In addition, the re-use of a syringe by people addicted to injectable drugs contributes to the spread of disease. Therefore, it is also desirable to have a needle that can only be used once to prevent drug users from sharing contaminated needles. Many improved syringes have been developed that provide for the covering of a needle after use. However, existing designs often have relatively complicated constructions. In addition, some of these so called "single-use" syringes can be returned to working order with minimal effort.
U.S. Pat. No. 4,973,316 shows a one-handed retractable safety syringe where the needle retracts into the medicine barrel by further depressing a plunger after the medicine has been injected. This overtravel disengages two triggers, thereby releasing a spring that pushes the needle holder and needle back into the barrel. U.S. Pat. No. 4,978,343 shows a similar structure but including a pimple on the needle holder that rocks the needle after retraction into the housing to prevent re-extension. In both patents the release mechanism is located inside the medicine barrel near the needle end. Such a configuration is relatively frail since it must be small enough to fit inside the medicine barrel. Moreover clearing a jam in the inaccessible trigger mechanism is difficult with an internal mechanism. In addition, having moving parts inside the barrel where the medicament is held could cause contamination if chips or fragments break off.
U.S. Pat. No. 5,049,133 shows a single-use safety syringe with a needle mounted in a head that is held by teeth-like triggers. When a plunger reaches an extreme position the triggers release the needle and head which then retract into the hollow cavity in the plunger under the urging of a compression spring. Again, the trigger mechanism is internally located inside the medicine barrel at the needle end. Accordingly, the mechanism would be prone to jamming and fragment contamination.
In international patent PCT/EP89/01408, the needle of a single-use syringe is held in a working position by a retaining member. After use, the retaining member is released by further pressing the plunger which allows a spring to retract the needle into the body. This rather complicated device requires the cooperation of a large number of internally located moving parts having the disadvantages noted above.
Other patents, not analyzed in detail herein, have a releasable needle shield, and may require two hands to lock the needle in the working position before use and to release the needle after use. The needle may be locked into place either by rotating the shield or operating a button or clip. See U.S. Pat. Nos.: 4,900,311, 5,013,301, 4,850,977, 4,927,416, 4,929,237, 4,955,868 and 5,049,136.
Accordingly there is a need for an effective safety syringe employing a retractable needle that is reliable, simple to clear if jammed, and unlikely to suffer from fragment contamination.